Hydraulically actuated spool valves employing a reduced drive area at the spool end are known. One such arrangement is disclosed in the aforementioned copending patent application and the prior art referred to therein. In such constriction a separate piston and cylinder means is employed as the motor to reciprocate the spool rather than employing the spool end and its bearing as the piston and cylinder. This enables the piston diameter to be smaller than the spool end diameter so that a greater stroke is achieved for a given volume of hydraulic fluid supplied to the drive motor. The separate piston is kept in contact with the spool end by a spring. Separation of the motor piston and the spool makes it unnecessary for these parts to be coaxial, thus saving on machining costs. The volume between the spool end and drive piston is enclosed so that it can be supplied with lubricant, e.g. system hydraulic fluid. Since the volume varies, to prevent hydraulic lock and cavitation, the volume is vented to reservoir, and/or in the case of the usual valve in which both ends of the spool are driven, to the volume at the opposite end of the spool.
Although the known construction operates satisfactorily, there is still a certain amount of wear adjacent the spool end, and its bearing and the piston which it would be desirable to reduce. In addition, especially in applications such as for hydraulic drive of an earth shaker for seismic exploration, e.g. "Vibrosis", it is desirable to reduce the mass of the moving parts, liquid and solid, in order that the valve motion can more precisely follow the dictates of the electric control signal applied to its pilot valve.